The bolted joint is a very important fastener method in modern engineering assemblies. It works by screwing together two or more parts with a bolt and nut. The bolt or nut may be made integral with one of those parts, and the screwing action draws the bolt and nut together so that their faces produce a clamping force. During clamping the bolt material can stretch or the material forming the parts being fastened together may compress, as the nut is tightened. It is very difficult to measure the precise extent of the above stretching or compression, and therefore to deduce the resulting clamping force. Experiments are therefore performed with force washers to arrive at a torque value which is easy to measure, so as to establish that the clamping force is between specified limits. Once that torque value has been established, it may be replicated as a reliable means of creating a bolted joint (a so-called “test joint”) with known characteristics, but to replicate reliably the amount of torque imparted during tightening of a known joint, it becomes essential that the rotary fasteners used to tighten up the test joints are also periodically checked, to make sure they are set up correctly before they are used on an assembly line.
International standards have been set up to specify performance test routines for checking the calibration of rotary tools which are used on assembly lines before fastening bolts and similar threaded fasteners. These performance test methods use Joint Rate Simulators (JRSs). These JRSs simulate the torque pattern that is experienced as a joint is tightened. To a first approximation, as a typical joint is tightened, the torque increases linearly with the angle turned by the screw thread. A JRS uses this characteristic to provide a test piece on which the tool will fit, such that when the tool applies torque to turn the test piece, that torque increases with the angle through which the bolt turns.
The rate of increase of torque with increasing angle is referred to as the torque-rate. A joint with a high torque-rate is referred to as a “hard joint”, and full tightening is generally accomplished in a fraction of a revolution. In contrast, in a low torque-rate joint (known as a “soft joint”) the full tightening is usually accomplished over a much greater angular range of movement, possibly several complete revolutions of fastener.
Test joints are known in which a rotatable shaft is physically braked, with the braking effort increasing as a function of rotation. The braking effort, which can be achieved either by brake shoes engaging the outer cylindrical surface of the shaft or by brake pads engaging opposite surfaces of a brake disc carried by the shaft, can be varied to simulate either a hard or a soft joint. Our own W098/10260 is an example of such a variable rate JRS. It allows the test joint parameters to be easily changed, allowing any test joint to be simulated; and it allows the torque to be removed after the joint has been tightened, so that a subsequent cycle of the performance testing routine can take place without any time delay. Any complete performance testing routine comprises a number of repeated tightening cycles of the test joint, with the results being averaged or statistically analyzed. This and other prior JRSs do not, however, have a moment of inertia that is matched to that of the real joint which they are simulating. The moment of inertia of the JRS is invariably greater than, and frequently vastly greater than, that of the real joint.
The disparity between the moment of inertia of the JRS and that of the real joint which it simulates increases when the mechanism for braking the test joint involves calliper brake pads braking against opposite sides of a brake disc. Disparities between the moment of inertia of the test joint and the moment of inertia of the real joint become particularly important when the test joint is used for the performance testing of impulse drive tools. These tools rely on the transfer of pulses of torque, each pulse being a few milliseconds in duration, with many pulses per second being applied to the joint. If the joint has a large moment of inertia, then the tool cannot transfer enough energy to make the joint initially free-turn before the joint tightens, and the tool can then stall. All JRSs with disc brakes suffer from this specific problem, and even JRSs with drum brakes clamping against opposite sides of a shaft can have moments of inertia that are not matched to that of the real joint under simulation, and so will not necessarily give true results for impulse tools.
The invention of our own WO2008/099204 provides a variable torque-rate test joint which has a working moment of inertia that is more closely matched to that of a screw-threaded bolt which it simulates. That compliance between the moment of inertia of the JRS and that of the joint under simulation is achieved by using, for the rotary component of the JRS to be driven by the rotary tool, a screw threaded bolt, the size and inertia of which can be accurately matched with that of the bolt of the joint under simulation.
The test joint according to WO2008/099204 utilizes a screw-threaded nut and bolt on a test rig, the bolt being adapted to be tightened by a rotary tool under test against a variable joint supported by the nut. The variable joint comprises a spring beam which extends in cantilever over a pivot point to engage beneath a head of the bolt. If the pivot point is close to the bolt head then that simulates a hard joint, whereas if the pivot point is more distant from the bolt then the joint becomes progressively more soft. The end of the spring beam remote from the bolt is anchored, either fast to the test rig or fast to the corresponding end of a second spring beam which extends in cantilever past the same pivot point to engage with a nut into which the bolt is screwed. For test purposes the bolt is screwed down against the variable joint, with the applied torque being carefully monitored over the whole angular range of progressive tightening of the bolt. At the end of the tightening process the bolt must be screwed back to its start position and the tightening process is repeated. A typical test sequence would involve a statistically significant number of repeated tightening processes. WO2008/099204 discloses a cam arrangement for the release of the joint compression at the end of each tightening sequence, to permit a rapid run-back of the tightened bolt after each monitored tightening sequence.
The release cams disclosed in WO2008/099204 have proved unsatisfactory when the variable torque-rate test joint simulates a very massive bolt such as may be included in constructional ironwork or in very heavy machinery. Inevitably the cams of WP2008/099204 are sent slightly over-centre to lock the distal ends of the spring beams, so that they do not disengage accidentally. With the heavier duty bolts used in some heavy industries, the pressure of the tightened bolt may be so great that it becomes very difficult or impossible to release those cams at the end of the test sequence, because their release requires the cams to pass once again over-centre to move to their release positions, with an attendant momentary small increase in the pressure applied by the test joint. It is therefore an object of the invention to create a variable torque-rate test joint which has an easily operable release mechanism which is suitable for use over the entire range of test bolt sizes, from small to even the most massive threaded joints.